1. Field of the Invention
The present invention relates to a recording apparatus which includes a process of forming electrostatic latent images on a changed recording medium by means of irradiation of laser beams, and more particularly, to a recording apparatus which is capable of recording multi-colored information on the recording medium with a plurality of laser beams.
2. Description of the Prior Art
A recording apparatus of the above kind includes, as shown, for instance, in FIG. 73, a drum-shaped photosensitive body 100 as the recording medium. In the periphery of the photosensitive body 100 there are arranged successively along the direction of rotation shown by the arrow, a first charger 101, a first exposure unit 102, a first developing unit 103, a second charger 104, a second exposure unit 105, a second developing unit 106, a transfer-stripping charger 107, a cleaner 110, and a discharger 109. One cycle of process is completed by electrically charging the photosensitive body 100 uniformly with the first charger 101, forming a second electrostatic latent image by the second exposure section 105, visualizing a second color by the second developing unit 106, carrying out a control processing if needed to equalize the amount of charges by the two color toners, though not shown, transferring dichromatic information onto a transfer material 108 with the transfer-stripping charger 107, cleaning with the cleaner 110 the toner that remains on the photo sensitive body 100 after transfer, and erasing the latent images with the discharger 109.
However, the existing apparatus has the second developing unit 106 which is of contact development type so that even when there is formed a first toner image 103a which is brought out to be visible, for example, by the first developing unit 103 as shown in FIG. 74(A), there may occur a case in which a portion of the first toner image 103a is scraped off by the second developing unit 106 as shown in FIG. 74(B). Then, in response to the exposure condition of the second exposure section 105, second toner 106a may be piled up by the second developing unit 106 over the first toner image 103a as shown in FIG. 74(C).
On the other hand, when the first toner 103a that was scraped off by the second developing unit 106 is sent into the inside of the second developing unit 106 to be mixed with the second toner 106a as shown in FIG. 75, the life of the developer (consisting of a carrier and a toner) will undergo a sharp reduction.
Further, in the case of the dichromatic printing process in which both of the first developing unit 103 and the second developing unit 106 are operated in the normal development mode, the changes in the surface potential of the photosensitive body 100, the conditions of the toner on the photosensitive body 100, and so forth will change as illustrated in FIG. 76(A).
Namely, due to charging by the first charger 101, the surface potential of the photosensitive body 100 is raised, and when the normal exposure is given using the first exposure section 102, only the information zone which is irradiated by the laser beam is maintained at a high potential to form an electrostatic, latent image, leaving the outside of the information zone at a low potential. The electrostatic latent image is brought out to be visible using a negatively charged toner by the first developing unit 103. When the photosensitive body 100 is charged again in this state by the second charger 104, the surface potential of the photosensitive body 100 returns to nearly the level of the first charged state, and the surface toner on the electrostatic latent image is transformed to a state in which it is charged positively by the appended charges.
Next, when the photosensitive body 100 is exposed normally by the second exposure section 105, there is formed an electrostatic latent image with high potential in the information zone, and at the same time there remains the image that was visualized in the past by the first developing unit 103. Further, an electrostatiic latent image is brought out visible by the second developing unit 106 in a second exposure using negatively charged toner. A small amount of the toner is attached also to the electrostatic latent image due to the first exposure.
The electrostatic latent image that is brought out to be visible in this manner by the two normal development modes is transferred onto the transfer material 108.
In addition, in the case of the dischromatic printing process in which the first developing unit 103 is operated in the inverted development mode and the second developing unit 106 is operated in the normal development mode, the surface potential of the photosensitive body 100 due to charging by the first charger 101 is raised, and an inverted exposure is carried out by the first exposure section 102 as shown in FIG. 76(B), bringing the information zone alone in low potential to form an electrostatic latent image, with the area outside of the information zone maintained at high potential. The electrostatic latent image is brought out to be visible by the first developing unit 103 due to positively charged toner. When the photosensitive body 100 is charged in this state again by the second developing unit 104, the surface potential of the photosensitive body 100 returns to approximately the level of the first charging.
Next, when the photosensitive body 100 is exposed normally by means of the second exposing section 105, the information zone becomes an electrostatic latent image with high potential, and the image that was brought out visible by first developing unit 103 remains as is. Then, the electrostatic latent image due to the second exposure is brought out visible by the second developing unit 106 with negatively charged toner, and a small amount of the toner is attached also to the electrostatic latent image due to the first exposure. After carrying out a pre-transfer charging with a charger which is not shown in order to give the same polarity to the electrostatic latent images that are brought out visible in this manner by the inverted development mode and the normal development mode, each of the electrostatic latent images that are brought out to be visible is transferred onto the transfer material 108.
In the case of the conventional dichromatic printing process by the combination of the normal-normal development modes or the dichromatic printing process by the combination of the inverted-normal development modes, there is necessarily involved a process of charging a toner with the charge that has the polarity that is opposite to the polarity of the toner.
In particular, in the dichromatic printing process by the combination of the inverted-normal development modes, the polarity of the toner used varies for each development mode so that there is an inconvenience in that in order to transfer simultaneously both electrostatic latent images that are brought out to be visible onto the transfer material 108, there has to be given a pre-transfer charging to invert the polarity of one of the toners. Moreover, when the dichromatic printing process is employed in which development is carried out in the inverted mode after development in the normal mode, there also arises the necessity of carrying out a pre-transfer charging.
Furthermore, in the dichromatic printing process of the combination of the normal-normal development modes, the toner polarity is the same in each of the developing units. However, it is inevitable to have the opposite charge on the toner, at the time of recharging with the second charger 104, as shown in FIG. 76(A).
When the opposite charge appears on the toner, although each image is transferred later with corona of respective polarity, it is clear that the efficiency for each is lower than that for the ordinary monochromatic transfer.
However, when a high resistance is given to the toner in order to enhance the transfer efficiency and to secure a stable development in a humid atmosphere, there arises a problem that the toner which sits on the photosensitive body inverts the polarity so that it is difficult to invert the polarity even with the reversed charging.
In addition, when the thickness of the toner layer on the photosensitive body is large, the toner layer is laminated in multiple layers rather than in a single layer. In such a case, when the top layer in particular is inverted, it prevents the transfer of the opposite charge to the inner toner layers, so that there is a problem that the toner polarity in the lower layers is difficult.
Moreover, the existing color copier in practice is of the type in which an image is transferred onto a transfer paper or an intermediate transfer drum for each color, and this process is repeated, to complete the full color print, so that this method can also be applied to the recording apparatus of the type under consideration.
However, in that case, the copying speed will have to be reduced sharply since a sheet of copy is obtained by repeating the process similar to the above.
Furthermore, in the existing recording apparatus of the above kind, when printing is done in only one color, if, for instance, while the apparatus is in printing operation in a first color, a second color is designated, then the printing operation in the second color will be initiated by temporarily interrupting the rotational driving of the photosensitive body simultaneous with the completion of printing operation in the first color. Therefore, the copying speed will have to be reduced in some cases.